The present invention relates to an exhaust gas purifier of an engine and a controller thereof.
According to a prior art, in order to remove toxic contents in the exhaust gas of an engine, for example, unburnt hydro carbon compound (HC), carbon monoxide (CO) and nitrogen oxides (NOx), there has been disclosed an exhaust gas purifier that is equipped with a catalytic converter rhodium installed in an exhaust pipe and converts them to non-toxic components by catalytic action.
With this purifier, however, since the catalytic action depends upon the catalyst temperature, the catalytic action cannot be performed until the catalyst temperature increases after the engine has started. Thus, there has been disclosed another exhaust gas purifier that have the toxic contents in the exhaust gas absorbed into the catalyst up to a certain volume temporarily and then purifies them under a temperature higher than the specified (Japanese Patent Application Laid-open No. Hei 05-79319 (1993)).
With the prior art as above, however, exhaust gas remains in the exhaust pipe and exhaust gas purifier after the engine has stopped. Because of this, there has been a problem that water content in the residual exhaust gas liquefies and adheres onto the surface of the catalyst, causing another problem that the catalyst temperature increase is delayed due to the decrease of the contact area between the catalyst and exhaust gas and vaporization of the adhered water content at the next engine start and so the purification performance lowers. Besides, if a sudden change in the catalyst temperature is caused, temperature difference is generated between the portions with and without adhered water and the catalyst may possibly break due to thermal stress.